Flaw Size Dependent Contrast Reduction and Additional Unsharpness by Scattered Radiation in Radiography Film and Digital Detectors in Comparison
Radiographic image quality depends by its classical definition on contrast, noise (film: granularity as equivalent to noise) and unsharpness. These parameters are usually measured with image quality indicators (IQI). The scattered radiation reduces always the image quality in radiography. The different effects were analyzed to explain differences between applications of X-ray film, imaging plates (computed radiography) and digital detector arrays (DDA). In textbooks and standard guides (e.g. ASTM E1000) the scatter ratio k and different contrast definitions are given. This is for instance the radiographic contrast, specific contrast and the relative contrast. For digital media additional parameters are introduced as signal/noise ratio (SNR) and contrast/noise ratio (CNR), whereas the inverse CNR is the well known contrast sensitivity (CS), which is typically measured with IQIs. The optimum conditions for film replacement are investigated in the European project FilmFree. In this context the influence of the scattered radiation is investigated as it is generated by the object and in the detector and its cassettes or casings. X-ray modeling (Monte Carlo technique) is used to calculate separately the effect of primary radiation and scattered radiation leaving the object. Scattered radiation produced in the object and detector yields an image, which is the overlap of the projection by the primary radiation and a scatter image with higher unsharpness. This effect becomes dominant especially at higher radiation energies. Such scatter images are characterized by unsharpness values in the cm range. This disturbs classical radiography as well as digital radiography and computed tomography. Furthermore, an additional contrast reduction due to scatter effects is observed for small flaws, if the detector is in contact with the object. This effect depends on the flaw size and the distance between detector and object as well as the intermediate filtering.
Film digital radiography computed radiography image quality scatteredradiation scatter contrast scatter unsharpness.
Uwe EWERT Uwe ZSCHERPEL Carsten BELLON Gerd-Rüdiger JAENISCH J(o)rg BECKMANN Mirko JECHOW
Institute for Materials Research and Testing (BAM), D-12200 Berlin, Germany;
国际会议
第十七届世界无损检测会议(17th World Conference on Nondestructive Testing)
上海
英文
342-352
2008-10-25（万方平台首次上网日期，不代表论文的发表时间）